1. Cross Reference to Related Applications
This application is related to application Ser. No. 262,849 entitled "CIRCUIT BOARD MOUNTING ARRANGEMENT FOR USE IN CYLINDRICAL INSTRUMENT HOUSING", Ser. No. 262,848 entitled "DISPLAY CONNECTION SCHEME FOR MODULAR ANALOG/DIGITAL INSTRUMENT", and Ser. No. 262,850 entitled "SCREW-ON POINTER FOR ELECTRICAL MEASURING INSTRUMENT", filed simultaneously with the present application, and assigned to the assignee of the present application.
2. Field of the Invention
The invention relates to the field of indicating instruments, and more particularly to a modular backlighted analog/digital aerospace instrument.
3. Description of the Prior Art
Aircraft and aerospace instruments have traditionally comprised one or more analog meter movements mounted within a housing and coupled to a pointer disposed adjacent an analog dial having graduated markings formed thereon. For convenience in reading such instruments in darkness or under poor conditions of illumination, these instruments are often provided with some form of backlighting or edge-lighting to illuminate the dial and/or pointer.
While such analog instruments have found widespread use throughout the aerospace industry such instruments have a major drawback in that an analog instrument must be interpreted by the human observer. Such observations are prone to error, especially in view of the literally dozens of such instruments which must be constantly scanned in modern commercial and military aircraft.
In order to obviate these problems there has been a demand for more precise display of critical operating parameters such as can be provided by digital displays. For human engineering purposes, it is generally desirable to combine an analog display with a digital display of a critical operating parameter.
Severe environmental constraints are placed on any proposed analog/digital instrument design. Such instruments must be rugged, accurate and reliable even when subjected to extremes of temperature, pressure, and vibration as is common in aircraft and aerospace environments. In addition, military and commercial standards dictate that most panel instruments be circular in shape and fit within standardized openings (diameter and depth) in an instrument panel. A problem arises in that the large number of electronic components needed to drive a digital display or used for signal conditioning of an analog display will often fill the space available behind the mounting panel without leaving sufficient room for the analog (mechanical) movement. The complexity of components and lack of space within the instrument housing also make it difficult to assemble the digital display and analog meter movement together without interference with each other and to readily interconnect components on jacent circuit boards. Such difficulties increase manufacturing and repair costs. In addition, the complexity of such displays makes it difficult to adapt an instrument to different lighting requirements and display types, e.g. light emitting diode (LED), liquid crystal (LCD), incandescent displays which are edge-illuminated, backlighted or unilluminated, for example.
There is thus a need for a compact and modular analog/digital instrument which is adaptable to various types of displays and lighting techniques and in which the digital display can be assembled or disassembled without interference to the analog meter movement in order to facilitate manufacturing, and to thus reduce assembly and repair costs.